「非鑄造」從（無效*）和間接引用到字符數組

Question

我幾乎完成了我在哪裏需要使用並行線程的家庭作業。我已經想出了pthreads。我留下的唯一問題是弄清楚如何通過pthread_create（）將多個參數傳遞給線程。

我需要兩個字符傳遞給線程。我必須將它們轉換爲（* void）以與pthread_create（）一起使用。我可以通過它們，但我不知道如何從函數中的*參數中獲取值。

void *my_function(void *parameter) { 

    /* ATTEMPT 1 - DOESN'T WORK */ 
    //char* arguments[2]; 
    //arguments = (char * [2]) parameter; 
    /*Error message: 
    error: ISO C++ forbids casting to an array type char* [2] [-fpermissive] 
    error: incompatible types in assignment of char** to char*[2] 
    */ 

    /* ATTEMPT 2 - DOESN'T WORK */ 
    //char *my_data = (char *)parameter; 
    //my_data is blank when I try to use cout to check it's values 

    /* What I need to do is get those two chars from the array and print them out as part of this thread function */ 

    pthread_exit(NULL); 
} 

int main(int argc, char **argv) { 

    char duration = '5'; //in reality, this value is taken from argv but I am leaving that out for brevity 

    pthread_t threads[3]; 

    for(int i=0; i < 3; i++){ 

     char thread_args[2] = {i, duration}; 

     //create thread with arguments passed in 
     int results = pthread_create(&threads[i], NULL, my_function, (void *) &thread_args); 

     //testing for pthread error 
     if (results){ 
      printf("ERROR; return code from pthread_create() is %d\n", results); 
      exit(-1); 
     } 

    } 

    /* Wait for all threads to complete */ 
    for (int j=0; j < num_threads; j++) { // https://computing.llnl.gov/tutorials/pthreads/ 
     pthread_join(threads[j], NULL); 
    } 


    /* some information prints here that is unrelated to my problem (the date, time, etc) */ 

    pthread_exit(NULL); 
} 

我能夠通過一個值沒有問題。有什麼建議麼？

最接近的現存的問題，我能找到的就是這一點，但我還是感到有沒有運氣：Converting from void* to char ** in C

謝謝！

Answer 1

注意，在這個循環中：

for(int i=0; i < 3; i++){ 
    char thread_args[2] = {i, duration}; 
    int results = pthread_create(&threads[i], NULL, my_function, (void *) 
    ... 
} 

thread_args是本地陣列自動存儲時間，生存時間，是依賴於每次迭代所以有一個機會，在這裏你存儲這些參數的內存可能是在線程訪問它之前釋放，這將導致未定義的行爲在這種情況下。

更好的方法是創建一個結構，你會使用到的數據傳送到這個線程：

typedef struct { 
    char duration; 
    int num; 
} ThreadData; 

那麼你的代碼看起來是這樣的：

void *my_function(void *parameter) { 
    // retrieve and print the thread data: 
    ThreadData* td = (ThreadData*) parameter; 
    printf("num = %d, duration = %c\n", td->num, td->duration); 
    delete td; 
    return NULL; 
} 

int main(int argc, char **argv) { 
    char duration = '5'; 
    pthread_t threads[3]; 

    for (int i = 0; i < 3; i++) { 
     // create structure that will be passed to thread: 
     ThreadData* td = new ThreadData; 
     td->duration = duration; 
     td->num = i; 

     //create thread with arguments passed in: 
     int ret = pthread_create(&threads[i], NULL, my_function, (void *) td); 

     //testing for pthread error: 
     if (ret) { 
      printf("ERROR; return code from pthread_create() is %d\n", ret); 
      exit(-1); 
     } 
    } 
    // wait for all threads to complete: 
    for (int i = 0; i < 3; i++) { 
     pthread_join(threads[i], NULL); 
    } 
    exit(0); 
} 

還要注意是結束線程的執行最好是使用return比pthread_exit，因爲與return可以保證線程程序中的變量將被銷燬，棧會被展開。有關詳細信息，請參閱return() versus pthread_exit() in pthread start functions